Since the end of the 20th century the cellular telephone industry has had enormous development in the world. From the initial analogue systems, such as those defined by the standards AMPS (Advanced Mobile Phone System) and NMT (Nordic Mobile Telephone), the development has during recent years been almost exclusively focused on standards for digital solutions for cellular radio network systems, such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications). Different digital transmission schemes are used in different systems, e.g. time division multiple access (TDMA) or code division multiple access (CDMA). Currently, the cellular technology is entering the so-called 3rd generation, providing several advantages over the former, 2nd generation, digital systems referred to above. Among those advantages an increased bandwidth will be provided, allowing effective communication of more complex data. The 3rd generation of mobile systems has been referred to as the UMTS (Universal Mobile Telephony System) in Europe and CDMA2000 in the USA, and is already implemented in Japan to some extent. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
One evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others have been approved for the Cellular hyperband (e.g., D-AS (IS-136)). Other frequency bands in which these devices will be operating include GPS (operating in the 1.5 GHz range) and UMTS (operating in the 2.0 GHz range). Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analogue modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedance and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna's length is to be minimised, another choice is the helical antenna. In addition, mobile terminal manufacturers encounter a constant demand for smaller and smaller terminals. This demand for miniaturisation is combined with desire for additional functionality such as having the ability to use the terminal at different frequency bands and different cellular systems.
It is commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in the 1500 MHz, 1800 MHz, 1900 MHz, 2.0 GHz and 2.45 GHz regions. Accordingly, antennas which provide adequate gain and bandwidth in a plurality of these frequency bands will need to be employed in portable terminals. Several attempts have been made to create such antennas.
In order to reduce the size of the portable radio terminals, built-in antennas have been implemented over the last couple of years. The general desire today is to have an antenna, which is not visible to the customer. Today different kinds of patches are used, with or without parasitic elements. The most common built-in antennas currently in use in mobile phones are the so-called planar inverted-F antennas (PIFA). This name has been adopted due to the fact that the antenna looks like the letter F tilted 90 degrees in profile. Such an antenna needs a feeding point as well as a around connection. If one or several parasitic elements are included nearby, they can be either grounded or dielectrically separated from ground. The geometry of a conventional PIFA antenna includes a radiating element, a feeding pin for the radiating element, a ground pin for the radiating element, and a ground substrate commonly arranged on a printed circuit board (PCB). Both the feeding pin and the ground pin are arranged perpendicular to the ground plane, and radiating element is suspended above the ground plane in such a manner that the ground plane covers the area under the radiating element. This type of antenna, however, generally has a fairly small bandwidth in the order of 100 MHz. In order to increase the bandwidth for an antenna of this design, the vertical distance between the radiating element and the PCB ground has to be increased, i.e. the height at which the radiating element is placed above the PCB is increased. Another solution to this problem is to add a dielectric element between the antenna and the PCB, in order to make the electrical distance longer than the physical distance.
U.S. Pat. No. 6,326,921 to Ying et al discloses a built-in, low-profile antenna with an inverted planar inverted F-type (PIFA) antenna and a meandering parasitic element, and having a wide bandwidth to facilitate communications within a plurality of frequency bands. A main element is placed at a predetermined height above a substrate of a communication device and the parasitic element is placed on the same substrate as the main antenna element and is grounded at one end. The feeding pin of the PIFA is proximal to the ground pin of the parasitic element. The coupling of the meandering, parasitic element to the main antenna results in two resonances. These two resonances are adjusted to be adjacent to each other in order to realise a broader resonance encompassing the DCS (Digital Cellular System), PCS and UMTS frequency ranges.
Today, the concept of built-in antennas is well known and extensively used by the mobile phone manufacturers. However, it is a fairly new concept, and the performance of such antennas is still a problem when even wider band capabilities are desired. Consequently, prior art antenna designs will still be a limiting factor when developing radio terminals with adequate bandwidth to cover plural bands, such as for example AS, EGSM (Extended GSM), DCS and PCS. A more general problem with built-in antennas is not only small band width, but also significantly worse gain performance than a traditional external antenna i.e. some kind of stub antenna.